In the production process of a semiconductor device, a product to be polished such as a wafer is polished using a polishing composition.
For example, a wafer is produced by polishing (which hereinafter may be referred to as polishing) a thin circular plate-shaped silicon or the like as a product to be polished that is obtained by slicing a single crystal ingot of silicon or another compound semiconductor, lapping it, and etching it.
Further, also when a fine electrical circuit is formed on a surface of the wafer, the surface of the wafer on which the circuit is formed is polished. In such a polishing step, polishing is performed generally using a polishing composition containing abrasive grains.
The surface of the product to be polished after polishing has to be clean, particularly in a step of finally polishing the silicon, even a slight amount of contamination on the surface of the wafer after the polishing is required to be eliminated.
In order to facilitate the removal of foreign matter that is one of the causes of such a contamination after the polishing such as residual abrasive grains and polishing waste on the surface of the wafer, a hydrophilic water soluble polymer that is hydrophilic with respect to the surface of the wafer is mixed in the polishing composition.
For example, Patent Literature 1 discloses a polishing composition using hydroxyethyl cellulose as the hydrophilic water soluble polymer.
However, hydroxyethyl cellulose has a large molecular weight, and tends to cause aggregation of abrasive grains due to its molecular structure when it is mixed in the polishing composition, which is a problem. Generally, the polishing composition is filtered before use. Since the polishing composition in which hydroxyethyl cellulose is mixed is easily clogged, a coarse filter needs to be used for its filtering. Thus; it is difficult to remove fine aggregate particles. When polishing is performed while such aggregate particles are present, the surface of the wafer is damaged, thereby causing surface defects (LPD: Light point defects). Further, since hydroxyethyl cellulose is a polymer derived from natural products, it is difficult to obtain a stable quality, which is another problem.
In order to suppress the aggregation of abrasive grains, it has been conceived that the surfaces of abrasive grains are subjected to a chemical surface treatment, thereby making it difficult for the abrasive grains to aggregate with each other in the polishing composition.
For example, Patent Literatures 2 and 3 disclose that the dispersibility of the abrasive grains in the polishing composition is enhanced by performing a surface treatment using silane or the like.
Patent Literature 4 discloses colloidal silica subjected to a surface treatment with aluminate ions, boric acid, or the like, or colloidal silica in which the surface potential is controlled.
However, even in the case of using the abrasive grains subjected to such a surface treatment, it is difficult to suppress their aggregation when they are present in the polishing composition together with hydroxyethyl cellulose.
Further, as a cause of the contamination of the wafer, there is metal contamination in which a slight amount of metal components derived from materials of the polishing composition or the wafer adheres onto the wafer, other than the residual abrasive grains and polishing waste. Such metal contamination after the polishing causes defects called pits on the surface of the wafer. However, it is difficult to reduce the metal contamination in conventional polishing compositions.